1. Field of the Invention
The present invention relates to a substrate for epitaxial growth employed as the substrate for various types of semiconductor devices.
2. Description of the Background Art
An Al-containing group III nitride film is employed as a semiconductor film constituting a semiconductor light-emitting device or a semiconductor photoreceptor or an underlayer for growing this semiconductor film. In recent years, the Al-including group III nitride film has also been remarked as a semiconductor film constituting an electronic device such as a high-speed IC chip employed for a portable telephone or the like or a shortwave LED.
The aforementioned semiconductor device is obtained by forming a buffer layer consisting of a group III nitride on a single-crystalline base consisting of a single crystal of sapphire or the like if necessary, thereafter forming an underlayer consisting of an Al-containing group III nitride film on the buffer layer and forming a group III nitride film having various target functions on the underlayer. The single-crystalline base, the buffer layer and the underlayer are generally regarded as an integrated substance, which is referred to as a substrate for epitaxial growth.
The buffer film, exhibiting a buffering effect of complementing the difference between the lattice constants of the base and the underlayer, is formed at a low temperature of 500 to 700° C. in a temperature range insufficient for ensuring crystallinity.
Another substrate for epitaxial growth may be obtained by forming a buffer film and an underlayer both consisting of AlN on an SiC base having excellent heat dissipation capacity and a lattice constant closer to that of a group III nitride film than a single crystal of sapphire. In this case, the buffer film and the underlayer are formed by MOCVD at a high temperature of at least 1000° C.
When a sapphire base is employed for preparing a substrate for epitaxial growth, the buffer film formed at a low temperature as described above exhibits low crystallinity. If an underlayer is formed on this buffer film for forming an Al-containing group III nitride film provided with various functions thereon, therefore, these films cannot be sufficiently improved in crystallinity. The buffer film has a relatively large quantity of dislocations, to result in a large quantity of dislocations also in the underlayer and the Al-containing group III nitride film formed thereon. Further, the buffer film and the Al-containing group III nitride film exhibit high mosaicity, which is evaluated on the basis of an X-ray rocking curve. In addition, the group III nitride film, particularly an Al-containing one, is disadvantageously remarkably dispersed in crystal quality such as crystallinity and dislocation density due to the temperature history from formation of the buffer film up to formation of the Al-containing group III nitride film.
In other words, the Al-containing group III nitride film provided with various functions cannot attain sufficiently high crystal quality in view of crystallinity and dislocation density when the sapphire base is employed for preparing the substrate for epitaxial growth. Thus, a semiconductor light-emitting device including this Al-containing group III nitride film is deteriorated in luminous efficiency, for example.
When an SiC base is employed for preparing a substrate for epitaxial growth, a buffer film or an underlayer of AlN having a thickness of about 0.1 μm or less is formed at a high temperature, while the film is insufficient in crystallinity with this degree of thickness. This film is also insufficient in surface flatness due to pitting or the like. Further, the film is disadvantageously readily influenced by defects present on the surface of the SiC base.